With you in the room, bacteria counts spike

A persons mere presence in a room can add 37 million bacteria to
the air every hour material largely left behind by previous
occupants and stirred up from the floor according to new research
by Yale University engineers.

We live in this microbial soup, and a big ingredient is our own
microorganisms, said Jordan Peccia, associate professor of
environmental engineering at Yale and the principal investigator of
a study recently published online in the journal Indoor Air. Mostly
people are re-suspending whats been deposited before. The floor
dust turns out to be the major source of the bacteria that we
breathe.

Many previous studies have surveyed the variety of germs present
in everyday spaces. But this is the first study that quantifies how
much a lone human presence affects the level of indoor biological
aerosols.

Peccia and his research team measured and analyzed biological
particles in a single, ground-floor university classroom over a
period of eight days four days when the room was periodically
occupied, and four days when the room was continuously vacant. At
all times the windows and doors were kept closed. The HVAC system
was operated at normal levels. Researchers sorted the particles by
size.

Overall, they found that human occupancy was associated with
substantially increased airborne concentrations of bacteria and
fungi of various sizes. Occupancy resulted in especially large
spikes for larger-sized fungal particles and medium-sized bacterial
particles. The size of bacteria- and fungi-bearing particles is
important, because size affects the degree to which they are likely
to be filtered from the air or linger and recirculate, the
researchers note.

Size is the master variable, Peccia said.

Researchers found that about 18 percent of all bacterial
emissions in the room including both fresh and previously deposited
bacteria came from humans, as opposed to plants and other sources.
Of the 15 most abundant varieties of bacteria identified in the
room studied, four are directly associated with humans, including
the most abundant, Propionibacterineae, common on human skin.

Peccia said carpeted rooms appear to retain especially high
amounts of microorganisms, but noted that this does not necessarily
mean rugs and carpets should be removed. Extremely few of the
microorganisms commonly found indoors less than 0.1 percent are
infectious, he said.

Still, understanding the content and dynamics of indoor
biological aerosols is helpful for devising new ways of improving
air quality when necessary, he said.

All those infectious diseases we get, we get indoors, he said,
adding that Americans spend more than 90 percent of their time
inside.

The researchers have begun a series of similar studies outside
the United States.

The papers lead author is J. Qian of Yale. Other contributors
are D. Hospodsky and N. Yamamoto, both of Yale, and W.W. Nazaroff
of the University of CaliforniaBerkeley.